Geoscience Reference
In-Depth Information
Fig. 7.12
Raised rock drumlin or flute on the platform at St.
Andrews, Scotland. The rock drumlin preserves sculptured S-forms
in
subsequently
modified
these
features.
Roller
vortices
have
cut
transverse troughs into the platform surfaces
the
form
of
muschelbrüche
and
cavettos.
Weathering
has
Fig. 7.13
Detached rock
drumlins or flutes lying en
echelon along the coastline at
MacDuff, Scotland. Plucking
dominates as the main
mechanism of erosion, but
smooth sculptured features such
as potholes and muschelbrüche
can be distinguished at a smaller
scale. The porthole has formed
through vortex formation on the
front and back of the stack. The
porthole and islet in the middle
point towards the headwall of the
Storegga slide
end abruptly in a well-developed cliff whose base is slightly
raised, about 2-3 m above the berm line of the modern
beach. While plucking dominates, the sides of some flutes
have been carved smoothly and the en echelon arrangement
is suggestive of helical flow under catastrophic flow.
Despite the irregular nature of the eroded bedrock surfaces,
smooth potholes appear on the seaward sides of some flutes.
These potholes lie above the present active wave zone. In
one case, vortex formation on the front and back of a stack
has carved out a porthole aligned towards the headwall of
the Storegga slides (Fig.
7.13
). Similar forms have been
linked to tsunami in eastern Australia, where vortices peel
Gardenstown and Cullen, isolated fluted stacks over 4 m
high are present in the middle of embayments (Fig.
7.14
).
The features are so remotely linked to the adjacent rock
coastline that in some cases they appear as stranded ero-
sional remnants in the middle of beaches. Their upper parts
often lie above the limit of present storm waves. Many
flutes still show evidence of cavettos along their sides. The
alignment of the flutes is structurally controlled, but the
features are best developed where the strike of the bedrock
points towards the headwall of the Storegga slides.
Tsunami also have molded headlands. For example, at
Logie Head, the end of the headland, which rises over 15 m
above present sea level, is separated from the main cliffline
by an erosional depression (Fig.
7.15
). The toothbrush-
shaped form is similar to the eroded headlands of southeast
Australia interpreted as a prominent signature of tsunami-
